APPLICATION NOTE—105
Introduction-1
INTRODUCTION
Application Note 105 December 2005 Current Sense Circuit Collection
Making Sense of Current Tim Regan, Editor
This Application Note Will Change Sensing and/or controlling current flow is a fundamental
requirement in many electronics systems, and the tech- niques to do so are as diverse as the applications them- selves. This Application Note compiles solutions to cur- rent sensing problems and organizes the solutions by general application type. These circuits have been culled from a variety of Linear Technology documents.
This Application Note is a growing and changing docu- ment. Many of the chapters listed below are placeholders for material that will be filled in soon. As the chapters are added, their links will be enabled.
Using the Application Note
Click the name of a chapter in the “Circuit Collection In- dex” below to open the PDF version of that chapter.
Circuits Organized by General Application
Each chapter collects together applications that tend to solve a similar general problem, such as high side cur- rent sensing, or negative supply sensing. The chapters are titled accordingly (see “Circuit Collection Index” be- low). In this way, the reader has access to many possible solutions to a particular problem in one place.
Contributors
Jon Munson, Alexi Sevastopoulos, Greg Zimmer, Michael Stokowski
, LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, µModule, Micropower SwitcherCAD, Multimode Dimming, No Latency ∆Σ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelPro- tect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products.
It is unlikely that any particular circuit shown will exactly meet the requirements for a specific design, but the sug- gestion of many circuit techniques and devices should prove useful. Specific circuits may appear in several chapters if they have broad application.
CIRCUIT COLLECTION INDEX
Level Shifting High Speed
Current Sense Basics
High Voltage Fault Sensing
High Side
Low Voltage Digitizing
Low Side
High Current (100mA to Amps) Current Control Negative Voltage
Unidirectional Low Current (Picoamps to Precision
Milliamps) Bidirectional
AC Motors and Inductive Loads
DC Batteries
Wide Range
APPLICATION NOTE 105: Current Sense Circuit Collection
Current Sense Basics
This chapter introduces the basic techniques used for sensing current. It serves also as a definition of common terms. Each technique has advantages and disadvan- tages and these are described. The types of amplifiers used to implement the circuits are provided.
To see other chapters in this Application Note, return to the Introduction.
LOW SIDE CURRENT SENSING
Current sensed in the ground return path of the power connection to the monitored load. Current generally flows in just one direction (uni-directional). Any switch- ing is performed on the load-side of monitor.
– +
ILOAD
ISENSE LOAD
OUTPUT ∝ ILOAD DC VSUPPLY
VCC
RSENSE
. Low Side Advantages
Low input common mode voltage Ground referenced output voltage Easy single supply design
Low Side Disadvantages
Load lifted from direct ground connection
Load activated by accidental short at ground end load switch
High load current caused by short is not detected Amplifier Types for Low Side Implementation
Precision zero-drift op amps: LTC2050, LTC2054 Instrumentation amplifiers: LTC2053, LT1990, LTC6943
Rail-to-Rail Input op amps: LT1677
HIGH SIDE CURRENT SENSING
Current sensed in the supply path of the power connec- tion to the monitored load. Current generally flows in just one direction (uni-directional). Any switching is per- formed on the load-side of monitor.
– +
ILOAD
ISENSE
LOAD
OUTPUT ∝ ILOAD DC VSUPPLY
RSENSE
High Side Advantages Load is grounded
Load not activated by accidental short at power con- nection
High load current caused by short is detected High Side Disadvantages
High input common mode voltages (often very high) Output needs to be level shifted down to system oper- ating voltage levels
Amplifier Types for High Side Implementation Dedicated current sensing amplifiers: LT6100, LTC6101, LT1787
Over-the-Top™ op amps: LT1637 Flying capacitor amplifier: LTC6943
Current Sense Basics-1
APPLICATION NOTE 105: Current Sense Circuit Collection
FULL-RANGE (HIGH AND LOW SIDE) CURRENT SENSING
Bi-directional current sensed in a bridge driven load, or unidirectional high side connection with a supply side switch.
– +
ILOAD
ISENSE OUTPUT ∝ ILOAD DC VSUPPLY
VCC
RSENSE LOAD
Full-Range Advantages
Only one current sense resistor needed for bidirec- tional sensing
Convenient sensing of load current on/off profiles for inductive loads
Full-Range Disadvantages
Wide input common mode voltage swings
Common mode rejection may limit high frequency accuracy in PWM applications
Amplifier Types for Bi-directional Implementation Difference amplifiers-LT1990, LT1991, LT1995, LT1996
Instrumentation amplifiers: LTC2053 Flying capacitor amplifier: LTC6943
SUMMARY OF CURRENT SENSE SOLUTIONS
The next few pages contain a table that summarizes cur- rent sense solutions and applicable devices. Look first in the “Type/Circuit” column and the “Gain” column for a general description of the application. Then scan across the other columns for applicable devices and their speci- fications.
Current Sense Basics-2
APPLICATION NOTE 105: Current Sense Circuit Collection
Current Sense Basics-3
ACCURACY SPEED
TYPE/CIRCUIT GAIN (V/V)
DEVICES AND PACKAGES
OFFSET VOLTAGE
(VOS)
INPUT CURRENT
(IBIAS)
BANDWIDTH SLEW RATE VSUPPLY
RANGE (VS) VIN RANGE (VCM)
DIFFERENTIAL VIN RANGE (SURVIVAL) High Side
One Direction Voltage Out
– +
– +
8
7
5 1
VS– LOAD
RG1 5k
VCC 2.7V TO 36V
RG2 5k
VIN (VCC + 1.4V) TO 48V
RO R
50k R/3
A4 6
A2 3
FIL 4
VEE VS+ RSENSE
VOUT
6100 F01
A1
A2 Q1
VO1 RE 10k
R 25k
2
10 to 50 LT6100
MSOP-8 DFN
300µV 5µA 100kHz 0.05V/µs 2.7V to 36V (VS + 1.4V) to 48V ±48V
High Side One Direction Current Out
– 4 +
3
5
2
1 IN–
V+
V– 10V
OUT
6101 BD
IN+
LTC6101/LTC6101HV
VBATTERY
IOUT VSENSE
RSENSE ILOAD
ROUT RIN
– +
L O A D
VOUT = VSENSE x ROUT RIN 5k
5k 10V
Resistor Ratio
LTC6101 LTC6101HV
SOT23-5 MSOP-8
350µV 350µV
250nA 250nA
200kHz 200kHz
2.5V/µs 2.5V/µs
4V to 70V 4V to 105V
(VS – 1.5V) to 70V (VS – 1.5V) to 105V
±70V
±105V
TION NOTE 105: Current Sense Circuit Collection APPLICA
Current Sense Basics-4
TYPE
ACCURACY SPEED
/CIRCUIT GAIN (V/V)
DEVICES AND PACKAGES
OFFSET VOLTAGE
(VOS)
INPUT CURRENT
(IBIAS)
BANDWIDTH SLEW RATE VSUPPLY
RANGE (VS) VIN RANGE (VCM)
DIFFERENTIAL VIN RANGE (SURVIVAL) High Side
Bi-directional
Current or Voltage (ROUT = 20k)
RSENSE
1787 F 01
RG2A 1.25k
RG2B 1.25k RG1A 1.25k
RG1B 1.25k
VOUT IOUT
VBIAS ROUT 20k VS–
– +
A1
Q1 Q2
CURRENT MIRROR VEE
FIL–
VS+
FIL+ ISENSE
Fixed 8 or Scaleable
LT1787 LT1787HV
SO-8 MSOP-8
75µV 75µV
20µA 20µA
300kHz 300kHz
0.1V/µs 0.1V/µs
2.5V to 36V 2.5V to 60V
2.5V to 36V 2.5V to 60V
±10V
±10V
High Side One Direction Voltage Out
Over the Top Amplifiers
– +
LT1637 3V TO 44V
3V R1 200Ω
RS 0.2Ω
R2 2k
VOUT (0V TO 2.7V) Q1 2N3904
1637 TA06
LOAD ILOAD
VOUT (RS)(R2/R1) ILOAD =
Resistor Ratio
LT1494 LT1636 LT1637 LT1672 LT1782 LT1783 LT1784
DIP-8 MS-8 SO-8 DFN SOT23-5 SOT23–6
150µV 50µV 100µV 150µV 400µV 400µV 1500µV
250pA 5nA 20nA 250pA
8nA 45nA 250nA
3kHz 200kHz
1MHz 12kHz 200kHz 1.25MHz
2.5MHz
0.001V/µs 0.07V/µs 0.35V/µs 0.005V/µs
0.07V/µs 0.42V/µs 2.4V/µs
2.1V to 36V 2.6V to 44V 1.8V to 44V 2.1V to 36V 2.2V to 18V 2.2V to 18V 2V to 18V
0 to VS + (36V – VS) 0 to VS + (44V – VS) 0 to VS + (44V – VS) 0 to VS + (36V – VS) 0 to VS + (18V – VS) 0 to VS + (18V – VS) 0 to VS + (18V – VS)
36V 44V 44V 36V 36V 36V 36V
APPLICATION NOTE 105: Current Sense Circuit Collection
Current Sense Basics-5
ACCURACY SPEED
/CIRCUIT GAIN (V/V)
DEVICES AND PACKAGES
OFFSET VOLTAGE
(VOS)
INPUT CURRENT
(IBIAS)
BANDWIDTH SLEW RATE VSUPPLY
RANGE (VS) VIN RANGE (VCM)
DIFFERENTIAL VIN RANGE (SURVIVAL) High Side
One Direction Voltage Out
Instrumentation Amplifier
VIN
2053 TA07
– +
1456 7 5V
LTC2053 3 8
2 0.1µF
0.1µF –5V
VOUT
VOUT = –VIN
Resistor Ratio
LTC2053 LTC6800
DFN MS-8
5µV 5µV
4nA 4nA
200kHz 200kHz
0.2V/µs 0.2V/µs
2.7V to 11V 2.7V to 5.5V
2.7V to 11V 2.7V to 5.5V
5.5V 5.5V
High Side or Low Side One Direction Voltage on a capacitor output Flying Capacitor
6943 • TA01b
0.01µF 9 POSITIVE OR
NEGATIVE RAIL
10 11
6 1µF RSHUNT
I
1/2 LTC6943 12
7
14 15
1µF E
E E
RSHUNT I =
Unity LTC6943 TSSOP – 16
6pA 90kHz 5V to 18V 5V to 18V 18V
TYPE
APPLICATION NOTE 105: Current Sense Circuit Collection
ACCURACY SPEED
TYPE/CIRCUIT GAIN (V/V)
DEVICES AND PACKAGES
OFFSET VOLTAGE
(VOS)
INPUT CURRENT
(IBIAS)
BANDWIDTH SLEW RATE VSUPPLY
RANGE (VS) VIN RANGE (VCM)
DIFFERENTIAL VIN RANGE (SURVIVAL) High Side or Low Side
Bi-Directional Voltage Out Difference Amplifiers
VIN– VIN+
VS+
VS– M9 M3 M1
P1 P3 P9
LT1991 8 9 10
1 2 3
7 6
5 4 R2*
10k
R1 10k
VIN+ – VIN– ILOAD = 10kΩ
*SHORT R2 FOR LOWEST OUTPUT OFFSET CURRENT. INCLUDE R2 FOR HIGHEST OUTPUT IMPEDANCE.
1 and 10 1 to 13
1 to 7 9 to 117
Pin Strap Configurable
LT1990 LT1991 LT1995 LT1996
SO-8 DFN MS–10
900µV 15µV 1000µV
15µV
2.5nA 2.5nA
105kHz 110kHz 32MHz 38kHz
0.55V/µs 0.12V/µs 1000V/µs 0.12V/µs
2.4V to 36V 2.7V to 36V 5V to 36V 2.7V to 36V
–250V to 250V –60V to 60V
0V to 36V –60V to 60V
±250V
±60V VS + 0.3V
±60V
Low Side One Direction Voltage Out Zero-Drift Amplifiers
– +
LTC2050HV 1 4 3
2050 TA08
5
2 5V
– 5V TO
MEASURED CIRCUIT
OUT 3V/AMP LOAD CURRENT IN MEASURED CIRCUIT, REFERRED TO –5V
10Ω 10k
3mΩ
0.1µF LOAD CURRENT
Resistor Ratio
LTC2050 LTC2054 LTC2054HV
SO-8 SOT23-5 SOT23 – 6
0.5µV 0.5µV 0.5µV
75pA 0.6pA 0.6pA
3MHz 500kHz 500kHz
2V/µs 0.5V/µs 0.5V/µs
2.7V to 7V 2.7V to 7V 2.7V to 12V
0V to (VS – 1.3V) 0V to (VS – 0.7V) 0V to (VS – 0.7V)
VS + 0.3V VS + 0.3V VS + 0.3V
Low Side One Direction Voltage Out
Rail to Rail I/O Amplifiers
+ –LT1800 0.1Ω
IL 0A TO 1A
VOUT 0V TO 2V
VOUT = 2 • IL f–3dB = 4MHz
UNCERTAINTY DUE TO VOS, IB < 4mA 3V
1k
1800 F02
52.3Ω 52.3Ω
Resistor Ratio
LT1218 LT1677 LT1800 LT1806 LT6200 LT6220
SO-8 DIP-8 SOT23-5 SOT23 – 6
25µV 20µV 75µV 100µV 1400µV
70µV
30nA 2nA 25nA
1µA 10µA 15nA
300kHz 7.2MHz 80MHz 325MHz 110MHz 60MHz
0.1V/µs 2.5V/µs 25V/µs 125V/µs
50V/µs 20V/µs
2V to 36V 2.5V to 44V 2V to 12.6V 1.8V to 12.6V 2.2V to 12.6V 2.2V to 12.6V
0V to VS 0V to VS 0V to VS 0V to VS 0V to VS 0V to VS
VS + 0.3V VS + 0.3V VS + 0.3V VS + 0.3V VS + 0.3V VS + 0.3V
Current Sense Basics-6
APPLICATION NOTE 105: Current Sense Circuit Collection
High Side
This chapter discusses solutions for high side current sensing. With these circuits the total current supplied to a load is monitored in the positive power supply line.
To see other chapters in this Application Note, return to the Introduction.
LT6100 Load Current Monitor
OUTPUT
VEE OUT
6100 F04
RSENSE
LT6100 8 1
VS– VS+ 2 A4
VCC
3 A2
4
7 C2
0.1µF
C1 0.1µF
3V
6
5 FIL
TO LOAD
+
5V
+
– +
This is the basic LT6100 circuit configuration. The inter- nal circuitry, including an output buffer, typically operates from a low voltage supply, such as the 3V shown. The monitored supply can range anywhere from VCC + 1.4V up to 48V. The A2 and A4 pins can be strapped various ways to provide a wide range of internally fixed gains.
The input leads become very hi-Z when VCC is powered down, so as not to drain batteries for example. Access to an internal signal node (pin 3) provides an option to in- clude a filtering function with one added capacitor. Small- signal range is limited by VOL in single-supply operation.
“Classic” Positive Supply Rail Current Sense
– +
LT1637 5V
200Ω
200Ω 0.2Ω
2k
0V TO 4.3V
1637 TA02
VOUT = (2Ω)(ILOAD) Q1 2N3904
LOAD ILOAD
This circuit uses generic devices to assemble a function similar to an LTC6101. A Rail-to-Rail Input type op amp is required since input voltages are right at the upper rail.
The circuit shown here is capable of monitoring up to 44V applications. Besides the complication of extra parts, the VOS performance of op amps at the supply is gener- ally not factory trimmed, thus less accurate than other solutions. The finite current gain of the bipolar transistor is a small source of gain error.
Over-The-Top Current Sense
– +
LT1637 3V TO 44V
3V R1
200Ω
RS 0.2Ω
R2 2k
VOUT (0V TO 2.7V) Q1
2N3904
1637 TA06
LOAD ILOAD
VOUT (RS)(R2/R1) ILOAD =
This circuit is a variation on the “classic” high-side cir- cuit, but takes advantage of Over-the-Top input capability to separately supply the IC from a low-voltage rail. This provides a measure of fault protection to downstream circuitry by virtue of the limited output swing set by the low-voltage supply. The disadvantage is VOS in the Over- the-Top mode is generally inferior to other modes, thus less accurate. The finite current gain of the bipolar tran- sistor is a source of small gain error.
High Side-1
APPLICATION NOTE 105: Current Sense Circuit Collection
Self-Powered High Side Current Sense
This circuit takes advantage of the microampere supply current and Rail-to-Rail input of the LT1494. The circuit is simple because the supply draw is essentially equal to the load current developed through RA. This supply cur- rent is simply passed through RB to form an output volt- age that is appropriately amplified.
High Side Current Sense and Fuse Monitor
OUTPUT 2.5V = 25A
VEE OUT
DN374 F02
RSENSE 2mΩ FUSE
LT6100 8 1 VS– VS+
BATTERY BUS
A4 ADC
POWER
≥2.7V
2 VCC
3 A2
4
7 C2
0.1µF
6
5 FIL
TO LOAD
– +
+
The LT6100 can be used as a combination current sensor and fuse monitor. This part includes on-chip output buff- ering and was designed to operate with the low supply voltage (≥2.7V), typical of vehicle data acquisition sys- tems, while the sense inputs monitor signals at the higher battery bus potential. The LT6100 inputs are toler- ant of large input differentials, thus allowing the blown- fuse operating condition (this would be detected by an output full-scale indication). The LT6100 can also be powered down while maintaining high impedance sense inputs, drawing less than 1µA max from the battery bus.
Precision High Side Power Supply Current Sense
–
+LTC6800
4 5 6
7 OUT
100mV/A OF LOAD CURRENT 10k
1.5mΩ
0.1µF 150Ω
6800 TA01
ILOAD 2 8
VREGULATOR
3
LOAD
This is a low-voltage, ultra-high-precision monitor featur- ing a Zero-Drift Instrumentation Amplifier (IA) that pro- vides Rail-to-Rail inputs and outputs. Voltage gain is set by the feedback resistors. Accuracy of this circuit is set by the quality of resistors selected by the user, small- signal range is limited by VOL in single-supply operation.
The voltage rating of this part restricts this solution to applications of <5.5V. This IA is sampled, so the output is discontinuous with input changes, thus only suited to very low frequency measurements.
Positive Supply Rail Current Sense
– +
1/2 LT1366 R1
200Ω
1366 TA01
LOAD ILOAD Rs 0.2Ω
R2 20k
Q1 TP0610L VCC
VO = ILOAD • RS = ILOAD • 20Ω
( )
– +
1/2 LT1366
R2 R1
This is a configuration similar to an LT6100 implemented with generic components. A Rail-to-Rail or Over-the-Top input op amp type is required (for the first section). The first section is a variation on the classic high-side where the P-MOSFET provides an accurate output current into R2 (compared to a BJT). The second section is a buffer to allow driving ADC ports, etc., and could be configured with gain if needed. As shown, this circuit can handle up to 36V operation. Small-signal range is limited by VOL in single-supply operation.
High Side-2
APPLICATION NOTE 105: Current Sense Circuit Collection
Precision Current Sensing in Supply Rails
6943 • TA01b
0.01µF 9 POSITIVE OR
NEGATIVE RAIL
10 11
6
1µF RSHUNT
I
1/2 LTC6943 12
7
14 15
1µF E
E E
RSHUNT I =
This is the same sampling architecture as used in the front-end of the LTC2053 and LTC6800, but sans op amp gain stage. This particular switch can handle up to 18V, so the ultra-high precision concept can be utilized at higher voltages than the fully integrated ICs mentioned.
This circuit simply commutates charge from the flying sense capacitor to the ground-referenced output capaci- tor so that under dc input conditions the single-ended output voltage is exactly the same as the differential across the sense resistor. A high precision buffer ampli- fier would typically follow this circuit (such as an LTC2054). The commutation rate is user-set by the ca- pacitor connected to pin 14. For negative supply monitor- ing, pin 15 would be tied to the negative rail rather than ground.
Measuring bias current into an Avalanche Photo Diode (APD) using an instrumentation amplifier.
CURRENT MONITOR OUTPUT 0mA TO 1mA = 0V TO 1V
+ –
35V
LT1789 A = 1
BIAS OUTPUT TO APD VIN
10V TO 33V
AN92 F02a
1k 1%
CURRENT MONITOR OUTPUT 0mA TO 1mA = 0V TO 1V
+ –
LT1789 A = 1
BIAS OUTPUT TO APD VIN
10V TO 35V
1N4684 3.3V
AN92 F02b
1k 1%
10M
The upper circuit uses an instrumentation amplifier (IA) powered by a separate rail (>1V above VIN) to measure across the 1kΩ current shunt. The lower figure is similar but derives its power supply from the APD bias line. The limitation of these circuits is the 35V maximum APD voltage, whereas some APDs may require 90V or more.
In the single-supply configuration shown, there is also a dynamic range limitation due to VOL to consider. The ad- vantage of this approach is the high accuracy that is available in an IA.
High Side-3
APPLICATION NOTE 105: Current Sense Circuit Collection
Simple 500V Current Monitor
Adding two external Mosfets to hold off the voltage al- lows the LTC6101 to connect to very high potentials and monitor the current flow. The output current from the LTC6101, which is proportional to the sensed input volt- age, flows through M1 to create a ground referenced output voltage.
Bidirectional Battery-Current Monitor
*OPTIONAL C2 1µF –5V
1787 F02
OUTPUT C3*
1000pF C1 1µF RSENSE
15V TO
CHARGER/
LOAD 1
2 3
4
8
7 6
5 LT1787 FIL+ FIL–
VBIAS
VOUT VS–
VS+
DNC
VEE
ROUT
This circuit provides the capability of monitoring current in either direction through the sense resistor. To allow negative outputs to represent charging current, VEE is connected to a small negative supply. In single-supply operation (VEE at ground), the output range may be offset upwards by applying a positive reference level to VBIAS (1.25V for example). C3 may be used to form a filter in conjunction with the output resistance (ROUT) of the part.
This solution offers excellent precision (very low VOS) and a fixed nominal gain of 8.
High Side-4
APPLICATION NOTE 105: Current Sense Circuit Collection
LTC6101 Supply Current
included as Load in Measurement
LTC6101
ROUT VOUT
6101 F06
3
5 4
2
1 RIN
LOAD V+
RSENSE
– +
This is the basic LTC6101 high-side sensing supply- monitor configuration, where the supply current drawn by the IC is included in the readout signal. This configu- ration is useful when the IC current may not be negligible in terms of overall current draw, such as in low-power battery-powered applications. RSENSE should be selected to limit voltage-drop to <500mV for best linearity. If it is desirable not to include the IC current in the readout, as in load monitoring, pin 5 may be connected directly to V+ instead of the load. Gain accuracy of this circuit is limited only by the precision of the resistors selected by the user.
Simple High Side Current Sense Using the LTC6101
DN374 F01
LT6101 4
LOAD BATTERY BUS
RSENSE 0.01Ω
RIN 100Ω
2
3
5
1 VOUT
4.99V = 10A
VOUT = ILOAD(RSENSE • ROUT/RIN) ROUT 4.99k
+ –
This is a basic high side current monitor using the LTC6101. The selection of RIN and ROUT establishes the desired gain of this circuit, powered directly from the battery bus. The current output of the LTC6101 allows it to be located remotely to ROUT. Thus, the amplifier can be placed directly at the shunt, while ROUT is placed near the monitoring electronics without ground drop errors.
This circuit has a fast 1µs response time that makes it ideal for providing MOSFET load switch protection. The switch element may be the high side type connected be- tween the sense resistor and the load, a low side type between the load and ground or an H-bridge. The circuit is programmable to produce up to 1mA of full-scale out- put current into ROUT, yet draws a mere 250µA supply current when the load is off.
High Side-5
APPLICATION NOTE 105: Current Sense Circuit Collection
High-Side Transimpedance Amplifier
Current through a photodiode with a large reverse bias potential is converted to a ground referenced output volt- age directly through an LTC6101. The supply rail can be as high as 70V. Gain of the I to V conversion, the trans- impedance, is set by the selection of resistor RL.
Intelligent High Side Switch
The LT1910 is a dedicated high side MOSFET driver with built in protection features. It provides the gate drive for a power switch from standard logic voltage levels. It pro- vides shorted load protection by monitoring the current flow to through the switch. Adding an LTC6101 to the same circuit, sharing the same current sense resistor, provides a linear voltage signal proportional to the load current for additional intelligent control.
High Side-6
APPLICATION NOTE 105: Current Sense Circuit Collection
48V Supply Current Monitor with Isolated Output and 105V Survivability
The HV version of the LTC6101 can operate with a total supply voltage of 105V. Current flow in high supply volt- age rails can be monitored directly or in an isolated fash- ion as shown in this circuit. The gain of the circuit and the level of output current from the LTC6101 depends on the particular opto-isolator used.
High Side-7
APPLICATION NOTE 105: Current Sense Circuit Collection
Low Side
This chapter discusses solutions for low side current sensing. With these circuits the current flowing in the ground return or negative power supply line is moni- tored.
To see other chapters in this Application Note, return to the Introduction.
“Classic” High-Precision Low Side Current Sense
– +
LTC2050HV 1 4
3
2050 TA08
5
2 5V
– 5V TO
MEASURED CIRCUIT
OUT 3V/AMP LOAD CURRENT IN MEASURED CIRCUIT, REFERRED TO –5V
10Ω 10k
3mΩ
0.1µF LOAD CURRENT
This configuration is basically a standard non-inverting amplifier. The op amp used must support common-mode operation at the lower rail and the use of a Zero-Drift type (as shown) provides excellent precision. The output of this circuit is referenced to the lower Kelvin contact, which could be ground in a single-supply application.
Small-signal range is limited by VOL for single-supply designs. Scaling accuracy is set by the quality of the user-selected resistors.
Precision Current Sensing in Supply Rails
6943 • TA01b
0.01µF 9 POSITIVE OR
NEGATIVE RAIL
10 11
6
1µF RSHUNT
I
1/2 LTC6943 12
7
14 15
1µF E
E E
RSHUNT I =
This is the same sampling architecture as used in the front-end of the LTC2053 and LTC6800, but sans op amp gain stage. This particular switch can handle up to 18V, so the ultra-high precision concept can be utilized at higher voltages than the fully integrated ICs mentioned.
This circuit simply commutates charge from the flying sense capacitor to the ground-referenced output capaci- tor so that under dc input conditions the single-ended output voltage is exactly the same as the differential across the sense resistor. A high precision buffer ampli- fier would typically follow this circuit (such as an LTC2054). The commutation rate is user-set by the ca- pacitor connected to pin 14. For negative supply monitor- ing, pin 15 would be tied to the negative rail rather than ground.
Low Side-1
APPLICATION NOTE 105: Current Sense Circuit Collection
–48V Hot Swap Controller
GND
OV UV
VEE VIN
SENSE SS
TIMER GATE
PWRGD DRAIN LTC4252-1 R1
402k 1%
R2 32.4k
1% CT
0.33µF CSS
68nF CC
18nF
–48V
RS 0.02Ω Q1 IRF530S VOUT
RC 10Ω R3 5.1k RIN
3× 1.8k IN SERIES 1/4W EACH
1
8 9 10 3
2 7 6 4
C1 5 10nF
CIN 1µF
CL 100µF
GND (SHORT PIN)
+
RD 1M
LOAD
EN
*
* M0C207
This load protecting circuit employs low-side current sensing. The N-MOSFET is controlled to soft-start the load (current ramping) or to disconnect the load in the
event of supply or load faults. An internal shunt regulator establishes a local operating voltage.
–48V Low Side Precision Current Sense
The first stage amplifier is basically a complementary form of the “classic” high-side current sense, designed to operate with telecom negative supply voltage. The Zener forms an inexpensive “floating” shunt-regulated supply for the first op amp. The N-MOSFET drain delivers a metered current into the virtual ground of the second stage, configured as a trans-impedance amplifier (TIA).
The second op amp is powered from a positive supply
and furnishes a positive output voltage for increasing load current. . A dual op amp cannot be used for this im- plementation due to the different supply voltages for each stage. This circuit is exceptionally precise due to the use of Zero Drift op amps. The scaling accuracy is estab- lished by the quality of the user-selected resistors. Small- signal range is limited by VOL in single-supply operation of the second stage.
Low Side-2
APPLICATION NOTE 105: Current Sense Circuit Collection
Fast Compact –48V Current Sense
–
+LT1797
0.1µF
R1 REDUCES Q1 DISSIPATION Q1
FMMT493
0.003Ω 1% 3W BZX84C6V8
VZ = 6.8V –48V SUPPLY (–42V TO –56V)
3.3k 0805
×3
30.1Ω 1%
ISENSE + –
R1 4.7k
VS = 3V 1k
1%
VOUT = 3V – 0.1Ω • ISENSE ISENSE = 0A TO 30A
ACCURACY ≈ 3%
–48V LOAD
1797 TA01
SETTLES TO 1% IN 2µs, 1V OUTPUT STEP
VOUT
This amplifier configuration is essentially the comple- mentary implementation to the classic high-side configu- ration. The op amp used must support common-mode operation at its lower rail. A “floating” shunt-regulated local supply is provided by the Zener diode, and the tran- sistor provides metered current to an output load resis-
tance (1kΩ in this circuit). In this circuit, the output volt- age is referenced to a positive potential and moves downward when representing increasing –48V loading.
Scaling accuracy is set by the quality of resistors used and the performance of the NPN transistor.
–48V Current Monitor
Low Side-3
APPLICATION NOTE 105: Current Sense Circuit Collection
In this circuit an economical ADC is used to acquire the sense resistor voltage drop directly. The converter is powered from a “floating” high-accuracy shunt-regulated supply and is configured to perform continuous conver- sions. The ADC digital output drives an opto-isolator, level-shifting the serial data stream to ground. For wider supply voltage applications, the 13k biasing resistor may be replaced with an active 4mA current source such as shown to the right. For complete dielectric isolation
and/or higher efficiency operation, the ADC may be pow- ered from a small transformer circuit as shown below.
–48V Hot Swap Controller
GND
OV UV
VEE VIN
SENSE SS
TIMER GATE
PWRGD DRAIN LTC4252-1 R1
402k 1%
R2 32.4k
1% CT
0.33µF CSS
68nF CC
18nF
–48V
RS 0.02Ω Q1 IRF530S VOUT
RC 10Ω R3 5.1k RIN
3× 1.8k IN SERIES 1/4W EACH
1
8 9 10 3
2 7 6 4
C1 5 10nF
CIN 1µF
CL 100µF
GND (SHORT PIN)
+
RD 1M
LOAD
EN
*
* M0C207
This load protecting circuit employs low-side current sensing. The N-MOSFET is controlled to soft-start the load (current ramping) or to disconnect the load in the
event of supply or load faults. An internal shunt regulator establishes a local operating voltage.
Low Side-4
APPLICATION NOTE 105: Current Sense Circuit Collection
Simple Telecom Power Supply Fuse Monitor
MOC207
MOC207
MOC207
FUSE STATUS
SUPPLY A STATUS
5V 47k
5V 47k
5V 47k
R3 47k 1/4W
SUPPLY B STATUS
OK: WITHIN SPECIFICATION OV: OVERVOLTAGE UV: UNDERVOLTAGE
–48V OUT
= LOGIC COMMON
0: LED/PHOTODIODE ON 1: LED/PHOTODIODE OFF
*IF BOTH FUSES (F1 AND F2) ARE OPEN, ALL STATUS OUTPUTS WILL BE HIGH SINCE R3 WILL NOT BE POWERED OUT F
–48V RETURN
VA 3
4
5 7
2 8 1
6 VB
FUSE A
F1 D1
F2 D2
RTN
LTC1921
FUSE B OUT A
OUT B
SUPPLY A –48V SUPPLY B –48V
R1 100k
R2 100k
SUPPLY A STATUS
0 0 1 1 VB OK UV OR OV
OK UV OR OV VA
OK OK UV OR OV UV OR OV
SUPPLY B STATUS
0 1 0 1
FUSE STATUS 0 1 1 1*
VFUSE B
= VB
≠ VB
= VB
≠ VB VFUSE A
= VA
= VA
≠ VA
≠ VA
The LTC1921 provides an all-in-one telecom fuse and supply-voltage monitoring function. Three opto-isolated
status flags are generated that indicate the condition of the supplies and the fuses.
Low Side-5
APPLICATION NOTE 105: Current Sense Circuit Collection
Negative Voltage
This chapter discusses solutions for negative voltage current sensing.
To see other chapters in this Application Note, return to the Introduction.
Telecom Supply Current Monitor
– +
LT6650 GND
IN OUT
FB
174k
20k 1nF
1µF VREF = 4V
1 2 3
2
56 7
4 1
8
4 5
LOAD
RS IL 48V
+
–
5V
VOUT
1990 AI01
–77V ≤ VCM≤ 8V VOUT = VREF – (10 • IL • RS)
LT1990
REF G1 G2
The LT1990 is a wide common-mode range difference amplifier used here to amplify the sense resistor drop by 10. To provide the desired input range when using a sin- gle 5V supply, the reference potential is set to approxi-
mately 4V by the LT6650. The output signal moves downward from the reference potential in this connection so that a large output swing can be accommodated.
–48V Hot Swap Controller
GND
OV UV
VEE VIN
SENSE SS
TIMER GATE
PWRGD DRAIN LTC4252-1 R1
402k 1%
R2 32.4k
1% CT
0.33µF CSS
68nF CC
18nF
–48V
RS 0.02Ω Q1 IRF530S VOUT
RC 10Ω R3 5.1k RIN
3× 1.8k IN SERIES 1/4W EACH
1
8 9 10 3
2 7 6 4
C1 5 10nF
CIN 1µF
CL 100µF
GND (SHORT PIN)
+
RD 1M
LOAD
EN
*
* M0C207
This load protecting circuit employs low-side current sensing. The N-MOSFET is controlled to soft-start the load (current ramping) or to disconnect the load in the
event of supply or load faults. An internal shunt regulator establishes a local operating voltage.
Negative Voltage-1
APPLICATION NOTE 105: Current Sense Circuit Collection
–48V Low Side Precision Current Sense
The first stage amplifier is basically a complementary form of the “classic” high-side current sense, designed to operate with telecom negative supply voltage. The Zener forms an inexpensive “floating” shunt-regulated supply for the first op amp. The N-MOSFET drain delivers a metered current into the virtual ground of the second stage, configured as a trans-impedance amplifier (TIA).
The second op amp is powered from a positive supply
and furnishes a positive output voltage for increasing load current. . A dual op amp cannot be used for this im- plementation due to the different supply voltages for each stage. This circuit is exceptionally precise due to the use of Zero Drift op amps. The scaling accuracy is estab- lished by the quality of the user-selected resistors. Small- signal range is limited by VOL in single-supply operation of the second stage.
Fast Compact –48V Current Sense
–
+LT1797
0.1µF
R1 REDUCES Q1 DISSIPATION Q1
FMMT493
0.003Ω 1% 3W BZX84C6V8
VZ = 6.8V –48V SUPPLY (–42V TO –56V)
3.3k 0805
×3
30.1Ω 1%
ISENSE
+ –
R1 4.7k
VS = 3V 1k
1%
VOUT = 3V – 0.1Ω • ISENSE ISENSE = 0A TO 30A
ACCURACY ≈ 3%
–48V LOAD
1797 TA01
SETTLES TO 1% IN 2µs, 1V OUTPUT STEP
VOUT
This amplifier configuration is essentially the comple- mentary implementation to the classic high-side configu- ration. The op amp used must support common-mode operation at its lower rail. A “floating” shunt-regulated local supply is provided by the Zener diode, and the tran- sistor provides metered current to an output load resis-
tance (1kΩ in this circuit). In this circuit, the output volt- age is referenced to a positive potential and moves downward when representing increasing –48V loading.
Scaling accuracy is set by the quality of resistors used and the performance of the NPN transistor.
Negative Voltage-2
APPLICATION NOTE 105: Current Sense Circuit Collection
–48V Current Monitor
In this circuit an economical ADC is used to acquire the sense resistor voltage drop directly. The converter is powered from a “floating” high-accuracy shunt-regulated supply and is configured to perform continuous conver- sions. The ADC digital output drives an opto-isolator, level-shifting the serial data stream to ground. For wider supply voltage applications, the 13k biasing resistor may be replaced with an active 4mA current source such as shown to the right. For complete dielectric isolation
and/or higher efficiency operation, the ADC may be pow- ered from a small transformer circuit as shown below.
Simple Telecom Power Supply Fuse Monitor
MOC207
MOC207
MOC207
FUSE STATUS
SUPPLY A STATUS
5V 47k
5V 47k
5V 47k
R3 47k 1/4W
SUPPLY B STATUS
OK: WITHIN SPECIFICATION OV: OVERVOLTAGE UV: UNDERVOLTAGE
–48V OUT
= LOGIC COMMON
0: LED/PHOTODIODE ON 1: LED/PHOTODIODE OFF
*IF BOTH FUSES (F1 AND F2) ARE OPEN, ALL STATUS OUTPUTS WILL BE HIGH SINCE R3 WILL NOT BE POWERED OUT F
–48V RETURN
VA 3
4
5 7
2 8 1
6 VB
FUSE A
F1 D1
D2 F2
RTN
LTC1921
FUSE B OUT A
OUT B
SUPPLY A –48V SUPPLY B –48V
R1 100k
R2 100k
SUPPLY A STATUS
0 0 1 1 VB OK UV OR OV
OK UV OR OV VA
OK OK UV OR OV UV OR OV
SUPPLY B STATUS
0 1 0 1
FUSE STATUS 0 1 1 1*
VFUSE B
= VB
≠ VB
= VB
≠ VB VFUSE A
= VA
= VA
≠ VA
≠ VA
The LTC1921 provides an all-in-one telecom fuse and supply-voltage monitoring function. Three opto-isolated
status flags are generated that indicate the condition of the supplies and the fuses.
